"A proton has a diameter of approximately one-millionth of a nanometer"

10−15 m

A proton is a nuclear particle that has a positive charge.This
charge is equal in magnitude to that of an electron, but opposite
in sign. Protons together with neutrons constitute all atomic
nuclei. Protons and neutrons together are regarded as nucleons.
The nuclear forces operating between a neutron and a proton are
much stronger than the electrostatic forces existing between atomic
and and molecular systems. The nuclear force between two nucleons
is considered separately from the electrostatic forces due to
electric charges that nucleons could carry. In addition mass,
charge, spin, and diameter play an important role in the mechanics
of the nucleus.

Protons are the nuclei of the simplest element -- hydrogen.
Protons also differentiate one element from the other because,
every different element has a different number of protons. Positively
charged hydrogen atoms were first identified as protons by J.
J. Thompson in 1906. He found that the electric charge on a proton
was equal but opposite that of an electron. However, the mass
of a proton he found to much larger than that of an electron.

The current estimate for the size of a proton was shown by
Rutherford in his scattering experiment in 1911. Rutherford showed
that the nucleus of the atom is very small and dense compared
to the rest of the atom. Its diameter was later found to be on
the order of 10−15 m.

The angular pattern and energy distribution of scattered electrons
gives us information about the internal structure of protons.
In 1963, M. Gell-Mann proposed that a proton was composed of three
spinning particles called quarks: two up quarks with a
charge of +2/3 e (positive two-thirds the electron's charge)
and one down quark with a charge of -1/3 e. This fractionally
charged quark concept was developed much further and has become
central to understanding the behavior and structure of protons.